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Light photons, color and energies of molecules.

  1. Nov 8, 2014 #1
    Hi, so I'm a first year neuroscience student at Carelton University in Canada. I had a little bit of a "revelation" with this topic recently after I understood it a bit better and I think this is really interesting. (If I understand it correctly!) We're learning about Kekule structures, conjugation (alternating single/double bonds in a molecule) and color.

    So basically what I understand is that, obviously, every wavelength of light has a different energy associated with it. This is the same for light photons. So when a light photon interacts with a molecule, it transfers its energy to the molecule and the photon is essentially "destroyed" or "absorbed". The length of the conjugate chain in the molecule plays a part in determining the wavelength of photon that gets absorbed by that molecule. So depending on how long the chain is, the difference in energy between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital is affected.

    What I understand is that when a photon (Let's pretend it's green) has roughly the same energy as the gap between the HOMO and LUMO, that photon will be absorbed by that molecule. This is because as the energy associated with the green photon bumps into an electron from the HOMO orbital and gives it energy to sit in the next orbital above it, LUMO. Now that the green photon just got absorbed we see all the other photons (or wavelengths of light) being reflected off of the object and we notice that there is no more green light being reflected off of the object. AKA we perceive that as "every color except green" or blue. (I think it's blue anyway).

    Now this was the main part of my question:
    If we turn off the lights in a room so that there is no visible light at all, do the objects that were originally absorbing green photons lose energy and have electrons drop back down to the HOMO orbital and lose their color? Essentially does this mean that in the dark, objects physically/chemically change and lose their color? Is this why we perceive shadows as being black, because an object is blocking photons from reaching the ground and since no (or less) photons are hitting the ground, it essentially has no color?

    This was quite a doozy to write, thanks a lot for reading this and any answers, opinions or further questions would be greatly appreciated. I can't find anything on this on the internet so I'm going to ask my chemistry professor after the weekend and I'll update this thread after I get an answer from him.

    Thanks again
  2. jcsd
  3. Nov 8, 2014 #2


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    Staff: Mentor


    If a tree falls in the forest... Colour is something perceived / interpreted by our visual system, stemming from the interaction of light with objects. If there is no light, it doesn't make sense to talk about colour. But I wouldn't call that "losing" a colour, as the same object will appear the same colour as soon as light is shone upon it again.

    The objects will not be in the same physical state, but in most cases there will not be any chemical change. That said, light can induce chemical changes, and in some cases lead to an actual (i.e., permanent) change in colour. Bleaching is one such phenomenon.

    When there is no light, there is no stimulation of the visual system, and our brain interprets this as black. That is, as far as I understand this, but you are the one studying neuroscience!
  4. Nov 8, 2014 #3
    Thanks for the answer, makes sense to me. And of course I understand that color is perceived by our brain and such, but I was thinking about this in more of a "particle interaction" sense. I also like the tree falling in the forest analogy too!

    I'll still be asking my chemistry professor to see what he says about this, but I get what you're saying, it's definitely more clear now.
  5. Nov 9, 2014 #4


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    I guess you need still to understand what happens to a molecule after absorption of a photon. Usually, the electronic degrees of freedom are strongly coupled to vibrational degrees of freedom of the molecule. Hence the electronically excited states decay on a sub-picosecond scale back to the electronic ground state with the molecule strongly vibrating (this is called a radiationless transition). Vibrations of a molecule is nothing else than heat, so that is the basic mechanism how substances heat up when left in the sun.
    So the molecule is basically all of the time in it's electronic ground state.
    There are only few exceptions: One are fluorescent materials where the coupling of the electronic and vibrational degrees of freedom is small. Hence these molecules can emit a photon when falling back to the ground state.
    This process is also very rapid.
    The other exception are phosphorescent materials. There the coupling of the excited electronic state to the ground state is very small, so that neither vibrational nor radiational processes are fast, and the radiation emitted when falling back to the ground state can be observed minutes or even hours after exitation. You certainly remember those gowing stars from your craddle.
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